1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which utilizes a magneto resistive effect.
2. Description of the Related Art
A magnetic random access memory which utilizes a tunneling magneto resistive effect (TMR) is characterized in that data is stored by using a magnetization state of an MTJ (Magnetic Tunnel Junction) element. This is described in, e.g., M. Durlam et al. “A low power 1 Mbit MRAM based on 1T1MTJ bit cell integrated with Copper Interconnects”, IEEE, 2002 Symposium on VLSI Circuits Digest of Technical Papers.
FIG. shows an example of a device structure of a conventional random access memory. FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1, and FIG. 3 is a cross-sectional view taken along the line III-III in FIG. 1.
An element isolation layer 12 having an STI (shallow Trench Isolation) structure is formed in a P type silicon substrate 11. An N-channel MOS transistor as a read selection switch RSW is formed in an element area surrounded by the element isolation layer 12. A gate of this transistor functions as a read word line RWL and extends in, e.g., an X-direction.
A source area 13A of the N-channel MOS transistor as the read selection switch RSW is connected with a read bit line RBL. The read bit line RBL extends in, e.g., a Y-direction and is connected with a read circuit (including a sense amplifier). A drain area 13B of the N-channel MOS transistor is connected with an electroconductive layer (e.g., a metal layer) 15 arranged above this transistor.
An MTJ element MTJ is arranged on the electroconductive layer 15. A write word line WWL which extends in the X-direction is arranged directly below the MTJ element MTJ. The write word line WWL is separated from the MTJ element MTJ by a fixed distance. A write current which is directed toward one direction flows through the write word line WWL at the time of writing.
A cap layer 16 formed of a conductor is arranged on the MTJ element MTJ. The MTJ element MTJ and the cap layer 16 both have, e.g., a square shape or a rectangular shape which is long in the X-direction. Here, the X-direction is a direction parallel with a magnetization easy axis of the MTJ element MTJ, and the Y-direction is a direction parallel with a magnetization hard axis of the MTJ element MTJ. The MTJ element MTJ and the cap layer 16 are covered with an insulating layer 14.
A write bit line WBL which extends in the Y-direction is arranged on the cap layer 16. The write bit line WBL has a damascene wiring structure, i.e., a structure arranged in a wiring groove of the insulating layer 17.
The write bit line WBL is electrically connected with the MTJ element MTJ through the cap layer 16. Yoke layers 18 and 19 are arranged on a top surface and side surfaces of the write bit line WBL. A write current which is directed toward one direction or the other direction flows through the write bit line WBL in accordance with a value of write data.
A consideration will now be given as to the MTJ element MTJ.
Currently, as the MTJ element MTJ, various kinds of layer structures and shapes are examined in order to improve its properties. For example, there have been examined a bottom pin structure in which a free layer (recording layer) is arranged on a pin layer (fixed layer), a top pin structure in which the pin layer is arranged on the free layer and a crisscross as a shape of the MTJ element MTJ.
FIG. 4 shows an example of the MTJ element MTJ.
This MTJ element MTJ is characterized in that a pin layer 21, a tunnel insulating layer 22 and a free layer 23 have the same shape.
The MTJ element MTJ is arranged on the electroconductive layer 15, and the cap layer 16 is arranged on the MTJ element MTJ. The cap layer 16 also has the same shape as that of the MTJ element MTJ. The write word line WWL is arranged directly below the MTJ element MTJ.
In such an MTJ element MTJ, for example, after the cap layer 16 is patterned, the free layer 23, the tunnel insulating layer 22 and the pin layer 21 are continuously etched with this cap layer 16 being used as a mask.
In this case, however, since a thickness of the MTJ element MTJ is small, etching and re-deposition of the electroconductive layer (e.g., a metal layer) 15 which serves as a substrate are produced when patterning the MTJ element MTJ, and an electroconductive layer 24 may be formed on each side wall of the MTJ element MTJ due to this re-deposition in some cases.
This electroconductive layer 24 short-circuits the pin layer 21 and the free layer 23 of the MTJ element MTJ, which becomes a factor of a bit defect.
In order to solve this problem, an MTJ element MTJ based on a so-called barrier stop technique has been examined.
FIG. 5 shows an example of an MTJ element MTJ based on the barrier stop technique.
This MTJ element MTJ is characterized in that a pin layer 21 and a tunnel insulating layer 22 have the same shape and a free layer 23 whose shape is different from that of the pin layer 21 is arranged on the tunnel insulating layer 22.
For example, the pin layer 21 and the tunnel insulating layer 22 which have the same shape as that of an electroconductive layer 15 are arranged on the electroconductive layer 15. The free layer 23 whose shape is different from that of the pin layer 21 is arranged on the tunnel insulating layer 22. On the free layer 23 is arranged, e.g., a cap layer 16 having the same shape as that of the free layer 23. A write word line WWL is arranged directly below the MTJ element MTJ.
In such an MTJ element MTJ, since the free layer 23, the tunnel insulating layer 22 and the pin layer 21 are not continuously etched with the electroconductive layer 15 being used as a substrate, the problem of so-called re-deposition can be solved.
In the barrier stop technique, however, patterning of the free layer 23 and patterning of the pin layer 21 are carried out by using different masks with different timings, and hence an alignment error is necessarily produced between them. A quantity of this alignment error varies depending on each chip or each wafer.
On the other hand, in the MTJ element MTJ based on the barrier stop technique, the pin layer 21 becomes larger than the free layer 23, and an influence given on the free layer 23 by a leak magnetic field generated from the pin layer 21 cannot be ignored. However, since the influence given on the free layer 23 by the leak magnetic field varies depending on a positional relationship between the free layer 23 and the pin layer 21, when an alignment error is generated between the free layer 23 an the pin layer 21 as described above, magnetic properties of the MTJ element MTJ also vary in accordance with a quantity of alignment error.
That is, irregularities are produced in magnetic properties of the MTJ element MTJ due to the alignment error at the time of lithography, and it is hard to obtain the MTJ element MTJ having stable properties.